DUBAI, United Arab Emirates — Syrian tanks were closing in on the rebel-held town of Qusair last month when a Kuwaiti sheik named Hajjaj al-Ajmi and his money machine roared into action. In a series of urgent messages on his Twitter account, Ajmi appealed for cash to help save the town’s defenders.

“I hope that we can be a means for helping them and relieving them,” the young cleric wrote to his 250,000 Twitter followers on May 25. He gave a phone number for making donations and asked readers to “kindly spread it.”

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A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

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The appeal came too late for the rebels in Qusair, but the technique has proved remarkably successful for Ajmi and a handful of other private backers of Syria’s patchwork of rebel groups. In just over a year, Ajmi’s foundation has raised hundreds of thousands of dollars to finance Syrian rebel groups.

U.S. and Middle Eastern officials describe the money as a small portion of a vast pool of private wealth being funneled to Syria’s warring factions, mostly without strings or oversight and outside the control of governments.

The private funding of individual militias — some with extremist views — further complicates the task facing the Obama administration as it ventures into arming Syria’s rebels. With its decision to increase support for the Syrian opposition, Washington is seeking to influence a patchwork of militia groups with wildly different abilities and views about how Syria should be run after the war.

The reluctance of Western governments to intervene over the past two years has allowed private donors to play an outsize role in shaping the Syrian conflict, officials say. From Persian Gulf cities hundreds of miles from the battlefield, wealthy patrons help decide which of Syria’s hundreds of rebel groups will receive money to pay salaries and buy weapons and supplies for the fight against the government of President Bashar al-Assad.

“Direct money from the gulf is super-empowering some of the jihadi groups,” said William McCants, a former adviser to the State Department and an expert on radical Islam. “With the United States holding back, there is a vacuum. And within this vacuum, private money is giving the jihadists more pull.”

So fierce is the competition for private funds that some Syrian groups adopt the language and dress of Islamists — growing beards, for example — to improve their chances with potential patrons, analysts say. Others post videos on YouTube thanking their gulf sponsors for past assistance and pleading for more.

A few have even named themselves after a gulf benefactor, like sports teams that adopt the logo of a corporate sponsor. One rebel group in eastern Syria now calls itself the “Hajjaj al-Ajmi Brigade,” in a tribute to the Kuwaiti sheik. A YouTube video posted by the group opens with a banner emblazoned with the sheik’s name and then shows a dozen masked fighters wearing camouflage fatigues and brandishing assault rifles.

“It’s anyone’s game,” said a U.S.-based Middle Eastern diplomat whose country has provided aid to some of the rebel factions opposed to Assad.

“Non-state actors are now involved in a big way. You see different players looking to create their own militias,” said the diplomat, who spoke on the condition of anonymity to discuss politically sensitive aid to the Syrian opposition. “It is beyond control.”

It is difficult to obtain reliable estimates of the amount of non-official aid given to Syrian groups. The donors are private citizens, and the deliveries typically take the form of cash-stuffed suitcases handed off to rebel emissaries at the Turkish border. Government experts and private analysts say the figure is certainly well into the millions of dollars. It is roughly the same pattern of private giving that funded the mujahideen fighters in Afghanistan and, years later, the militant Islamist movement that came to be known as al-Qaeda, analysts say.

Virtually all of the money from gulf states flows to anti-Assad forces that share a similar Sunni Arab background. Similar cash flows have bolstered pro-Assad forces in Syria, analysts say, including donations from Shiites in Iraq, Iran and Lebanon, mirroring the larger regional schism between the two major branches of Islam. Hezbollah, the Shiite militant group based in Lebanon, has provided fighters and training for Syrian government forces.

Clues about the impact of private giving can be gained from the YouTube and Facebook postings of several Syrian groups that acknowledged gifts with online thank-you notes. Last year, the Syrian Revolutionary Front, an Islamist organization with ties to the Muslim Brotherhood, acknowledged receiving nearly $600,000 from the Popular Commission to Support the Syrian People, a fund managed by Ajmi and another Kuwaiti sheik, Irshid al-Hajri.

Ahar al-Sham, considered one of the most radical of the Syrian Islamist militias, recorded a similar public thank-you for $400,000 the group says it received from the same fund. In its Web posting, the group specifically thanked Ajmi and Hajri, saying it “asks God to reward them and those behind them with the best of rewards.”

In an interview with the international Arab newspaper al-Hayat, an Ahar al-Sham official said private gifts are highly valued because they are not subject to government interference or corruption.

“The difference is that the aid that comes to us reaches us directly. As for the other factions, the aid they receive stops in Istanbul and does not reach Syria,” said the official, identified as Abu Zayd, the militia’s officer in charge of enforcing sharia law. He described the group’s principle backers as “Syrian expatriates in the gulf in addition to Arab and international charitable societies.”

Most of the private support comes as cash — usually dollars or euros. The money enables militias to buy whatever weapons are available on the region’s bustling black market, free of limits or restrictions attached to government money, analysts say.

In some cases, private donors have been directly involved in arranging arms shipments, said Asher Berman, a blogger and contributing writer for the Foreign Policy Research Institute who has researched Syria’s rebel factions.

“It’s all about obtaining weapons,” Berman said. “The Libyans particularly have a lot of weapons that can be directly transferred to Syria, and you hear about sheiks from the gulf arranging weapons purchases.”

The freewheeling nature of the private assistance has prompted attempts by gulf states such as Saudi Arabia and the United Arab Emirates to restrict donations to Syrian fighters by charities and wealthy individuals. But Sunni-led governments in Bahrain and, most notably, Kuwait, have largely declined to interfere with private fundraising efforts.

McCants, the former State Department adviser, attributed Kuwait’s prominent role to relatively weak terrorism finance laws and to the country’s large and politically connected community of Salafists, who practice an austere form of Islam.

Among the Kuwaitis, no one is more public about the Syria fundraising than Ajmi, scion of a prominent Kuwaiti family whose vast wealth was derived from oil and construction businesses. Ajmi and a small group of relatives and partners have aggressively promoted their Popular Commission charity on social media while making numerous trips to Syria to meet with leaders of favored rebel groups.

Ajmi’s Web postings have featured photos of the bushy-bearded sheik posing with Syrian rebel leaders, including the head of Liwaa al-Umma, a Syrian rebel group whose Web site calls for the establishment of “Islamic governance” in post-Assad Syria.

Ajmi, who did not respond to a request for an interview, has been unabashed in taking credit for his role in supporting the rebels. His prolific tweets include near-daily appeals for donations “for mujahid” — literally, “holy warriors” — in Syria, as well as for civilian victims of the civil war, which began as an uprising in March 2011. His online messages are often accompanied by photos of Syrian children killed or wounded in the fighting.

Mouaz Moustafa, director of the Syria Emergency Task Force, which supports humanitarian efforts in Syria, acknowledged that private donors have made positive contributions by helping deliver essential supplies to communities destroyed by fighting.

But, he added, the aid “becomes problematic when you see private groups deciding to arm different brigades. It undermines unity, and it hurts the opposition in the long run.”

Other analysts noted that the rebels already are badly and perhaps hopelessly fragmented, a problem for which many say the West deserves at least part of the blame. Mustafa Alani, a counterterrorism expert at the Dubai-based Gulf Research Council, said private donors became power brokers among rebel groups by default because the United States and other Western powers declined to support more moderate groups within the Syrian opposition.

“The Obama administration was always afraid that the wrong side would get the weapons,” ­Alani said. “But now we have a situation in which the wrong side already has them. And that side is self-supplying and self-financing.”

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